A conventional rotary internal combustion engine comprises a rotor which rotates and orbits within a casing. Mounted on the rotor are replaceable seals between the rotor and the casing which separate the combustion chambers. Because of the large number of high speed rotations and because the seals sweep the entire inner surface of the casing with each rotation, the seals are subjected to a great amount of wear; they must be replaced frequently.
The problem of seal wear has been helped substantially by the advent of rotors which do not rotate within the casing but merely orbit about the center of the casing. Since the seals do not sweep around the entire casing, but only move back and forth over a small area, seal wear is substantially reduced. This is described, for example, in U.S. Pat. No. 3,743,451 to Chapman.
Typically orbiting designs, however, tend to require special shapes for their housings and rotors which are expensive and difficult to manufacture.